Catalyst for polymerizing ethylene, comprising compounds of several transition metals, and a polymerization process using said catalyst

ABSTRACT

A catalyst for polymerizing ethylene, especially at high temperature and high pressure, comprising at least two halogen compounds of transition metals, one of which is a titanium trichloride syncrystallized with aluminum chloride. The catalyst has the formula 
     
         (TiCl.sub.3, 1/3AlCl.sub.3)(MX.sub.3).sub.x (MgX.sub.2).sub.y 
    
     wherein 0.3≦x≦3, 0≦y≦20, M is a transition metal of Groups V B and VI B of the Periodic System, and X is a halogen. The second transition metal is preferably vanadium, chromium, molybdenum, or tungsten. The catalyst is made by subjecting titanium trichloride syncrystallized with aluminum chloride, the trivalent halide of M, and, optionally, anhydrous magnesium halide to a milling step in which the milling energy is at least equal to three kWh per kg of solid material treated, which results in a binary solid solution structure Ti-M of which the dimension of crystallites is less than or equal to 100 Angstroms.

The present invention concerns catalysts for the polymerisation ofethylene and, more particularly, Ziegler type catalysts, having a solidsolution structure.

Numerous Ziegler type catalysts are already known for the polymerisationof ethylene and alpha-olefins. These catalysts generally comprise thecatalytic component itself, consisting of at least one halogen compoundof a transition metal of Groups IV to V of the Periodic System, and anactivator selected from the hydrides and organometallic compounds of themetals of Groups I to III of the Periodic System. The catalyticcomponent comprising the transition metal may be supported by an inertcarrier, such as alumina, silica, magnesium oxide, magnesium halidesetc.

BACKGROUND OF THE INVENTION

The present invention relates more particularly to Ziegler typecatalysts, comprising at least two halogen compounds of transitionmetals, one of which is a halogen compound of titanium, crystallisedtogether with a halogen compound of a metal of Group III of the PeriodicSystem, and the other of which is a halogen compound of a metal ofGroups V b and VI b of the Periodic System, these catalysts beingoptionally supported by an inert carrier.

The literature gives some examples of Ziegler type catalysts comprisingtwo halogen compounds of two different transition metals: for example,U.S. Pat. No. 3,288,769 describes catalytic components for polymerisingethylene in particular, having the formula (TiCl₃, VCl₃)_(x), x lyingbetween 0.25 and 4, and permitting a catalytic yield of 15 grams ofpolymer per milli-atom of titanium and vanadium per hour at 40° C.Similarly, U.S. Pat. No. 3,223,651 describes a catalyst of formula

    y TiCl.sub.3, (1-y) VCl.sub.3, 0.33 AlCl.sub.3,

y lying between 0.5 and 0.97, capable of producing a synergistic effectfor the polymerisation of alpha-olefins at a temperature below 100° C.and at a pressure below 35 bar.

On the other hand, it is known to polymerise ethylene at a pressure ofbetween 300 and 2500 bar, approximately, and at a temperature of between180° C. and 300° C., approximately. Within the framework of this kind ofprocess, it is desired to improve, on the one hand, the productionyield, related to the catalyst used, and, on the other hand, the qualityof the polymer produced by modifying the following parameters inparticular: density, molecular weight and distribution of molecularweights. It is therefore an object of the present invention to developcatalysts that can be used for polymerising ethylene in conditions ofhigh temperature and high pressure, as defined above, and capable ofimproving yield and quality of the polymer produced.

SUMMARY OF THE INVENTION AND DESCRIPTION OF PREFERRED EMBODIMENTS

The catalysts for the polymerisation of ethylene according to theinvention have the formula

    (TiCl.sub.3,1/3 AlCl.sub.3) (MX.sub.3).sub.x (MgX.sub.2).sub.y,

in which 0.3≦x≦3, 0≦y≦20, M is a transition metal of Groups V b and VI bof the Periodic System and X is a halogen. These catalysts have a binarysolid solution structure, Ti-M, which may be characterised by thedimension of its crystallites. It has been found that, from the point ofview of the efficiency of these catalysts, this dimension, determined bythe radiocrystallographic analysis method (Sherrer's Law) at rightangles to the plane (300), should preferably be less than, or equal to,100 Angstrom units (A.U.). As is understood by their expanded formula,these catalysts may, on occasion--when y>0--be supported by an inertcarrier, consisting of an anhydrous magnesium halide. Among the metalsM, vanadium and chromium are preferred, but molybdenum and tungsten canalso be used. The halogen of the magnesium halide and that of MX₃ may bedifferent or the same and are selected from fluorine, chlorine, bromineand iodine. The behaviour of the catalysts according to the inventionwith respect to the polymerisation of ethylene includes an importantaspect, which makes them particularly suitable for use in conditions ofhigh pressure and of high temperature. In fact, it has been observedthat, contrary to the catalysts of the prior art already cited, they donot show the synergy phenomenon of the production yield in conditions oflow pressure, but only in conditions of high pressure and temperature.

The preferred manufacturing process for the catalysts according to theinvention consists in bringing into contact titanium trichloride,crystallised together with aluminum chloride, the trivalent halide ofmetal M and, optionally, anhydrous magnesium halide for a sufficientlylong period of time for the dimension (determined as above) of thecrystallites of the solid solution obtained to be below 100 Angstromunits. This can be effectively achieved by submitting the threeabove-mentioned chlorides to a milling step, in which the milling energyis at least equal to 3 kWh per kg of solid material treated. Moreprecisely, it has been observed that the efficiency of these catalystsin the polymerisation of ethylene is the greater, the higher thismilling energy. However, with a view to obtaining optimum efficiency andhaving regard to operating costs and the need for economising energy, itis generally not necessary for the milling energy to be above 25 kWh,approximately, per kg of solid material treated.

It has been found, moreover, that, with regard to use in conditions oflow pressure and moderate temperature, it is desirable to choosecatalysts according to the invention, for which 2≦y≦20. This particularclass of catalysts according to the invention thus finds more variedapplication, because it can be employed in all processes for thepolymerisation of ethylene, whatever their conditions of temperature andpressure.

The present invention also relates to processes for the polymerisationof ethylene, using the catalysts defined above in selected conditions. Afirst process for the polymerisation of ethylene consists in bringingethylene, at a pressure of between 300 and 2500 bar, approximately, andat a temperature of between 180° and 300° C. approximately, into thepresence of a catalytic system comprising (a) at least one catalyst offormula (TiCl₃, 1/3AlCl₃)(MX₃)_(x) (MgX₂)_(y), in which 0.3≦x≦3, 0≦y≦20,M is a transition metal of Groups V b and VI b of the Periodic Systemand X is a halogen, and (b) at least one activator selected from thehydrides and organometallic compounds of metals of Groups I to III ofthe Periodic System, the atomic ratio of the activator metal to the sumTi+M lying between 0.1 and 10 and the mean residence time of thecatalytic system in the polymerisation reactor lying between 2 and 100seconds. This residence time depends on the temperature in the reactor,in the sense that it is the longer, the lower the temperature. Thisfirst process may bring into operation, especially when the temperatureand/or the pressure of polymerisation are not very high, the pressure ofan inert hydrocarbon, having preferably less than 5 carbon atoms, suchas, for example, propane or butane.

A second process for the polymerisation of ethylene according to theinvention consists in bringing ethylene, at a pressure of between 1 and200 bar, approximately, and at a temperature of between 20° and 200° C.,into solution or suspension in an inert liquid hydrocarbon having atleast 6 carbon atoms, preferably selected from among saturated aliphaticor cycloaliphatic compounds and aromatic compounds, in the presence of acatalytic system comprising (a) at least one catalyst of formula (TiCl₃,1/3AlCl₃)(MX₃)_(x) (MgX₂)_(y), in which 0.3≦x≦3, 2≦y≦20, M is atransition metal of Groups V b and VI b of the Periodic System and X isa halogen, and (b) at least one activator selected from among thehydrides and organometallic compounds of metals of Groups I to III ofthe Periodic System, the atomic ratio of the activator metal to the sumTi+M lying between 1 and 1000. In this type of process, the meanresidence time of the catalytic system in the polymerisation reactor is,generally, several minutes and can reach some hours.

If, in the high pressure process, an autoclave reactor or tubularreactor, having several reaction zones, is employed, it may beadvantageous, with a view to producing certain polymer grades, to adopta particular arrangement for the polymerisation equipment, such as, forexample, one of those described in U.S. Pat. Nos. 4,105,842 and4,168,356. Frequently, it will be useful for controlling the melt indexof the polymer, especially of polyethylene, to effect polymerisation inthe presence of a chain transfer agent, such as hydrogen. In the highpressure process, this agent will be used at the rate of from 0.04 to 2%by volume, related to ethylene.

Concerning the polymerisation or copolymerisation of ethylene, theprocess according to the invention permits the production of a wholerange of polymers, the density of which lies between 0.905 and 0.960g/cm³ and the melt index of which lies between 0.1 and 100 dg/min,approximately. Polymers of relatively low density, for example ofbetween 0.905 and 0.935 g/cm³, are obtained by copolymerising ethylenewith an alphaolefin having from 3 to 8 carbon atoms, for example propeneat the rate of from 15 to 35% by weight, or butene-1 at the rate of 15to 60% by weight.

The processes according to the invention make it possible to improve, onthe one hand, the production yield in relation to the catalyst employedand, on the other hand, the quality of the polymer produced, bymodifying the parameters of density, molecular weight and distributionof molecular weights. Other advantages of the invention will becomeclear on reading the following examples, given by way of illustrationand without implying any limitation.

EXAMPLES 1 TO 3

There are introduced in a discontinuous ball-mill, on the one hand,titanium trichloride, crystallised together with aluminiumchloride,--TiCl₃, 1/3AlCl₃,--and, on the other hand, chromiumtrichloride CrCl₃, (and, for Example 3, vanadium trichloride VCl₃), insuch quantities that the atomic ratios, Cr/Ti and V/Ti, are equal to thevalues indicated in Table I. After two hours of joint milling, thecatalyst obtained is dispersed in methyl cyclohexane and activated bydimetylethyldiethylsiloxalane in such quantity that the atomic ratio,Al/Ti, is equal to 6. Polymerisation of ethylene is theneffected,--optionally in the presence of propene comonomer in theproportion by weight, indicated in Table I,--in a continuous process ata pressure of 600 bar in an autoclave reactor, having a volume of 0.6liter and maintained at a temperature of 230° C., by injecting thecatalyst dispersion thus prepared in such a way that the mean residencetime of the catalyst in the reactor is equal to 30 seconds,approximately. Hydrogen is injected into the reactor in the quantityindicated in Table I, so as to control the melt index of the polymerproduced. Apart from the working conditions, Table I below indicates thecatalytic yield, CY, expressed in kilograms of polyethylene permilliatom of titanium, the melt index Mi, measured according to ASTMStandard D 1238 and expressed in dg/min, and the density ρ, expressed ing/cm³.

                  TABLE I                                                         ______________________________________                                        Example                                                                              Cr/Ti   V/Ti   % H.sub.2                                                                           % C.sub.3 H.sub.6                                                                    CY   Mi   ρ                            ______________________________________                                        1      0.33    0      0.5   0      3.0  3.6  0.948                            2      1       0      0.25  29.5   1.1  12.8 0.932                            3      0.33    0.67   0.25  29.5   3.1  3.2  0.931                            ______________________________________                                    

EXAMPLES 4 AND 5

There are introduced in a discontinuous ball-mill, on the one hand,titanium trichloride, crystallised together with aluminum chloride,TiCl₃, 1/3AlCl₃, and, on the other hand, vanadium trichloride, VCl₃, insuch quantities that the atomic ratio, V/Ti, is equal to the valueindicated in Table II. After two hours of joint milling, the catalystobtained is dispersed in methyl cyclohexane and activated bydimethylethyldiethylsiloxalane in such quantity that the atomic ratio,Al/Ti, is equal to 6. Ethylene is then polymerised in a continuousprocess at a pressure of 1200 bar in a cylindrical autoclave reactor,having a volume of 3 liters and divided, by means of baffle-plates, intothree identical zones, the first of which is maintained at a temperatureof 220° C. and the third of which is maintained at a temperature of 260°C. The temperature T₂ of the second zone varies with the examples and isindicated in Table II. The catalyst dispersion is injected into thereactor in such a way that the mean residence time of the catalyst inthe reactor is about 30 seconds. Hydrogen is injected into the reactorin the quantity indicated in Table II, so as to control the melt indexof the polyethylene produced. Apart from the working conditions, TableII below indicates the catalytic yield CY, expressed in kg of polymerper milli-atom of titanium and vanadium, the melt index MI, the densityρ as well as the number average molecular weight Mn and the percentage Bof molecular weights below 5,000, determined by gel chromatography.

EXAMPLES 6 AND 7

The identical procedure as in the experiments of Examples 4 and 5 isfollowed, except that the catalyst, introduced into the reactor, onlyconsists of titanium trichloride, TiCl₃, 1/3AlCl₃, for Example 6 andonly of vanadium trichloride, VCl₃, for Example 7. The different workingconditions and the polymerisation results are recorded in Table IIbelow.

                  TABLE II                                                        ______________________________________                                        Example  4       5           6     7                                          ______________________________________                                        V/Ti     1       2           0                                                T.sub.2 °C.                                                                     170     220         245   240                                        % H.sub.2                                                                              2.1     2.3         1.7   1.0                                        CY       10.1    7.7         3.4   3.0                                        MI       7.3     7.2         6.2   6.9                                        ρ    0.958   0.958       0.954 0.956                                      Mn       17 000  17 500      11 500                                                                              9 000                                      B        5.9     5.8         10.0  12.8                                       ______________________________________                                    

Thus it is clearly shown by a comparison of Examples 4 and 5, on the onehand, and 6 and 7, on the other, that the catalysts according to theinvention make it possible, for polymers of equivalent melt indices, toincrease considerably the catalytic yield as well as the molecularweight, while reducing the content of low molecular weight and raisingthe density.

EXAMPLES 8 TO 11

There are introduced in a discontinuous ball-mill, the specific power ofwhich is 0.8 kW per kilogram of material treated, equimolecularquantities of TiCl₃, 1/3AlCl₃ and of VCl₃. After a joint milling run ofduration t, expressed in hours, the catalyst obtained is submitted toradio-crystallographic analysis, which enables the dimension c of thecrystallites in the plane (3 0 0) to be determined according toScherrer's Law. It is observed that the catalyst has a solid solutionstructure. After this analysis, the catalyst is dispersed in methylcyclohexane and activated by dimethylethyldiethylsiloxalane in such aquantity that the atomic ratio, Al/(Ti+V), is equal to 6.

Ethylene is then polymerised in a continuous process by the same workingprocedure as in Example 4 and 5, the temperature T₂ being maintainedconstant at 230° C. Table III below indicates, apart from the millingtime for the catalyst, the yield CY, expressed as in the trials 4 to 7,the density ρ, the molecular weight Mn, the percentage B and thedimension c, expressed in Angstrom units.

                  TABLE III                                                       ______________________________________                                        Example t       CY      ρ Mn     B     c                                  ______________________________________                                        8       0       2.0     0.954 10 000 10.9  2 000                              9       4       7.1     0.958 15 000 7.3   100                                10      8       9.8     0.956 16 500 5.9   80                                 11      15      12.5    0.954 22 500 4.2   60                                 ______________________________________                                    

EXAMPLE 12

The catalyst, obtained in accordance with Example 9, is dispersed inmethyl cyclohexane and then activated by monofluorodiethylaluminium insuch a quantity that the atomic ratio, Al/(Ti+V), is equal to 6.Ethylene is then polymerised in a continuous process at a pressure of600 bar in the presence of 0.5 mole percent of hydrogen in a cylindricalautoclave reactor, having a volume of 0.6 liter and maintained at atemperature of 230° C. The catalyst dispersion is injected into thereactor in such a way that the mean residence time of the catalyst inthe reactor is of about 30 seconds. A polyethylene, having a melt indexequal to 8.2 dg/min, is then obtained with a catalytic yield of 6.1 kgof polymer per milli-atom of titanium and vanadium.

EXAMPLES 13 AND 14

Milling energy of 4 kWh per kg of material treated is applied in adiscontinuous ball-mill to a mixture of TiCl₃, 1/3AlCl₃ and VCl₃ in themolecular proportions indicated in Table IV. The catalyst obtained,dispersed in methyl cyclohexane and activated bydimethylethyldiethylsiloxalane at an atomic ratio, Al/(Ti+V)=6, is usedfor polymerising ethylene in a continuous operation at a pressure of 1200 bar in a cylindrical autoclave reactor, having a volume of 3 litersand maintained at a temperature of 240° C., the mean residence time ofthe catalyst in this reactor being about 30 seconds. Table IV belowindicates, apart from the atomic ratio V/Ti in the catalyst, thepolymerisation results, measured as for the preceding examples.

                  TABLE IV                                                        ______________________________________                                        Example  V/Ti      CY     ρ   Mn    B                                     ______________________________________                                        13       0.5       6.2    0.959   22 000                                                                              3.9                                   14       3         6.0    0.959   21 500                                                                              4.0                                   ______________________________________                                    

EXAMPLES 15 TO 17

Milling energy E, expressed in kWh per kg of material treated, isapplied in a discontinuous ball-mill to a mixture of TiCl₃, 1/3AlCl₃, ofVCl₃ and of MgCl₂ in the molecular proportions indicated in Table Vbelow. The catalyst obtained, dispersed in a C₁₁ -C₁₂ hydrocarbon cutand activated by dimethylethyldiethylsiloxalane at an atomic ratio,Al/(Ti+V)=100, is used for polymerising ethylene in solution in theabove-mentioned C₁₁ -C₁₂ cut in a steel autoclave reactor, having acapacity of 1 liter at a temperature of 200° C. and at a pressure of 6bar for the duration of one minute. The solution is subsequentlyrecovered and the polymer separated by filtration after cooling.

Table V below indicates, apart from the working conditions, thecatalytic yield CY, expressed in grams of polymer per gram of titaniumper minute and per atmosphere.

                  TABLE V                                                         ______________________________________                                        Example  V/Ti      Mg/Ti      E      CY                                       ______________________________________                                        15       1          6         4.0    606                                      16       1         12         6.1    678                                      17       2         12         3.1    474                                      ______________________________________                                    

EXAMPLES 18 TO 20

The catalyst, obtained in accordance with Example 15, is dispersed inmethyl cyclohexane and then activated by dimethylethyldiethylsiloxalane,(Examples 18 and 19), or by an equimolecular mixture ofmonochlorodiethylaluminium and trioctylaluminium, (Example 20) at anatomic ratio, Al/(Ti+V), equal to 6. Ethylene is then polymerised in thepresence of this catalytic system:

in a cylindrical autoclave reactor, having a volume of 0.6 liter andmaintained at a temperature of 230° C. and at a pressure of 600 bar, inso far as Example 18 is concerned;

in a cylindrical autoclave reactor, having a volume of 3 liters andmaintained at a temperature of 240° C. and at a pressure of 1 200 bar,in so far as Examples 19 and 20 are concerned;

the mean residence time of the catalyst in the reactor being about 30seconds. Table VI below indicates the polymerisation results andparticularly the yield CY, the density ρ, the molecular weight Mn andthe percentage B.

EXAMPLE 21

The catalyst, obtained in accordance with Example 17, dispersed inmethyl cyclohexane and then activated by dimethylethyldiethylsiloxalaneat an atomic ratio, Al/(Ti+V), equal to 6, is used for polymerisingethylene in a cylindrical autoclave reactor, having a volume of 3 litersand maintained at a temperature of 240° C. and at a pressure of 1 200bar, the mean residence time of the catalyst in the reactor being about30 seconds. Table VI below indicates the results of this polymerisation.

                  TABLE VI                                                        ______________________________________                                        Example  CY        ρ     Mn       B                                       ______________________________________                                        18       9.5       0.958     12 500   5.4                                     19       6.5       0.957     29 500   2.4                                     20       7.4       0.956     18 500   4.5                                     21       6.5       0.962     24 000   3.2                                     ______________________________________                                    

EXAMPLE 22

The catalyst, obtained in accordance with Example 16, dispersed inmethyl cyclohexane and activated by dimethylethyldiethyl, siloxalane atan atomic ratio, Al/(Ti+V), equal to 6, is used for copolymerisingethylene and propene, (30% by weight), in a cylindrical autoclavereactor, having a volume of 0.6 liter and maintained at a pressure of600 bar and at a temperature of 230° C., in the presence of 0.25 molepercent of hydrogen. A copolymer, having a density of 0.939 g/cm³ and amelt index of 7.2 dg/min, is produced with a catalytic yield of 4.1 kgper milli-atom of titanium and vanadium.

What we claim is:
 1. A catalyst for polymerizing ethylene comprising atleast two halogen compounds of transition metals, one of which is atitanium trichloride syncrystallized with aluminium chloride, having theformula:

    (TiCl.sub.3, 1/3AlCl.sub.3)(MX.sub.3).sub.x (MgX.sub.2).sub.y

wherein 0.3≦x≦3, 0≦y≦20, M is a transition metal of group V B and VI Bof thhe Periodic system and X is a halogen.
 2. A catalyst according toclaim 1, having a binary solid solutioh structure Ti-M of which thedimension of crystallites is less than or equal to 100 Angstrom units.3. A catalyst according to claim 1, wherein 2≦y≦20.
 4. A process formanufacturing a catalyst according to claim 1, 2, or 3, comprisingbringing into contact titanium trichloride crystallized together withaluminum chloride, the trivalent halide of metal M and, optionally,anhydrous magnesium halide, and subjecting said three halides to amilling step in which the milling energy is at least equal to 3 kWh perkg of solid material treated.
 5. A process according to claim 4, whereinthe milling energy is not above 25 kWh per kg of solid material treated.6. A catalyst according to claim 1, wherein M is selected from the groupconsisting of vanadium, chromium, molybdenum, and tungsten.
 7. Acatalyst according to claim 6, wherein M is vanadium.
 8. A catalystaccording to claim 6, wherein M is chromium.
 9. A catalyst according toclaim 1, wherein x is more than 1.0.
 10. A catalyst for polymerizingethylene comprising at least two halogen compounds of transition metals,one of which is a titanium trichloride syncrystallized with aluminumchloride, having the formula:

    (TiCl.sub.3, 1/3AlCl.sub.3)(MX.sub.3).sub.x (MgX.sub.2).sub.y

wherein 0.3≦x≦3, 0≦y≦20, M is a transition metal of Groups V B and VI Bof the Periodic System and X is a halogen, said catalyst being made by aprocess comprising bringing into contact titanium trichloridecrystallized together with aluminum chloride, the trivalent halide ofmetal M and, optionally, anhydrous magnesium halide, and subjecting saidthree halides to a milling step in which the milling energy is at leastequal to 3 kWh per kg of solid material treated.